Ptaquiloside (1) is a potentcarcinogen isolated from bracken fern. Under weakly alkaline conditions, the carcinogen is converted into dienone 2 which is thought to be the ultimate agent responsible for bracken fern carcinogenicity. This study details the selective alkylation and strand scission of DNA with dienone 2. Dienone 2 forms covalent adducts through N-3 of adenine or N-7 of guanine with opening